It is a common requirement, particularly in mobile handset design, for the various circuits involved to be as integrated as possible in order to reduce size, cost and weight. When, however, attempts have been made to integrate, or provide "on chip", a VCO using an external resonator, problems have arisen, particularly at UHF, of frequency hopping or mode switching. These problems arise because an integrated circuit package will always have associated parasitic inductance and capacitance associated with the external lead, lead frame and bond wires. It is possible for these parasitic components to dominate the on-chip components and so produce unwanted modes of oscillation. As the VCO gets close to the frequency at which the parasitic components resonate, the VCO may exhibit a tendency to hop to this frequency and become difficult to tune satisfactorily. An example of a known on-chip VCO will be described later.
Hitherto, it has been common practice to use a separate VCO module, but these use more expensive technology and packaging techniques. It is desirable to reduce the need for such external VCO modules, and thereby achieve a reduction in cost and increased reliability.